1.Field of the Invention
The present invention relates to silver catalysts for the oxidation of ethylene to ethylene oxide, and especially to the preparation of catalyst supports or carriers having improved properties such that catalysts comprising the carriers have enhanced utility.
2. Description of the Prior Art
Processes for the production of ethylene oxide involve the vapor phase oxidation of ethylene with molecular oxygen using a solid catalyst comprised of silver on a support such as alumina. There have been efforts by many workers to improve the effectiveness and efficiency of the silver catalyst for producing ethylene oxide. U.S. Pat. No. 5,051,395 provides an analysis of these efforts of various prior workers.
U.S. Pat. Nos. 3,962,136, 4,010,115 and 4.012,425 describe the use of alkali metal promoters such as cesium to improve silver ethylene oxide catalysts.
Among the many prior teachings in this area is that of U.S. Pat. No. 4,007,135 (see also UK 1,491,447) which teaches variously silver catalysts for the production of ethylene and propylene oxides comprised of a promoting amount of copper, gold, magnesium, zinc, cadmium, mercury, strontium, calcium, niobium, tantalum, molybdenum, tungsten, chromium, vanadium, and/or preferably barium, in excess of any present in immobile form in the preformed support as impurities or cements (column 2, lines 1-15), silver catalysts for the production of propylene oxide comprising a promoting amount of at least one promoter selected from lithium, potassium, sodium, rubidium, cesium, copper, gold, magnesium, zinc, cadmium, strontium, calcium, niobium, tantalum, molybdenum, tungsten, chromium, vanadium and barium, in excess of any present in immobile form in the preformed support as impurities or cements (column 2, lines 16-34), as well as silver catalysts for producing ethylene oxide or propylene oxide comprising (a) a promoting amount of sodium, cesium, rubidium, and/or potassium, and (b) magnesium, strontium, calcium and/or preferably barium in a promoting amount (column 3, lines 5-8).
U.S. Pat. No. 5,057,481, and related U.S. Pat. No. 4,908,343 are concerned with silver ethylene oxide catalysts comprised of cesium and an oxyanion of a group 3b to 7b element.
U.S. Pat. No. 3,888,889 describes catalysts suitable for the oxidation of propylene to propylene oxide comprised of elemental silver modified by a compound of an element from Group 5b and 6b. Although the use of supports is mentioned, there are no examples. The use of cesium is not mentioned.
European Patent 0 266 015 and U.S. Pat. No. 4,766,105 deal with supported silver catalysts promoted with rhenium and a long list of possible copromoters.
U.S. Pat. No. 5,102,848 deals with catalysts suitable for the production of ethylene oxide comprising a silver impregnated support also having thereon at least one cation promoter such as cesium, and a promoter comprising (i) sulfate anion, (ii) fluoride anion, and (iii) oxyanion of an element of Group 3b to 6b inclusive of the Periodic Table.
U.S. Pat. No. 5,486,628 describes a silver catalyst promoted with alkali metal, rhenium and a rare earth or lanthanide component.
U.S. Pat. No. 5,011,807 is concerned with an ethylene oxide catalyst comprised of silver, alkali metal, a transition metal, and sulfur on alumina support.
The support of choice in the preparation of silver ethylene oxide catalysts has in the past been a solid inorganic material such as alumina, silica, or titania based compounds, or combinations thereof. Alpha alumina which may contain silica has been an especially preferred carrier.
Various patents have focused on the pretreatment of such carriers to improve the utility thereof. U.S. Pat. No. 5,102,848, for example, shows repeated alpha alumina support washing with 90xc2x0 C. deionized water prior to deposition of the catalyst components. In the same patent, the carrier was also washed with HF solution at 25xc2x0 C. In both cases there was no claim or demonstration of the effect of carrier washing on the catalyst""s stability.
Later U.S. Pat. No. 6,103,916 similarly shows washing alpha alumina support with 90xc2x0 C. water repeatedly prior to deposition of the catalytic components in ethylene oxide catalyst preparation.
The prior art has disclosed that the presence of either sodium or lithium will have a profound effect on the performance of the silver catalyst. The claims of different patents, however, are not in agreement regarding the effect of these two alkali metals.
The prior art has been inconsistent about the effect of sodium on the catalytic performance of the silver catalyst. For instance, several patents have disclosed the importance of the presence of a minimum amount of Na on the surface of the carrier:
1. U.S. Pat. No. 4,740,493 states that the carrier should have at least 50 ppm of soluble Na ion, see claims 1 and 5.
2. U.S. Pat. No. 4,414,135 states, in the first claim, the advantage of a catalyst containing at least 1000 ppm Na, in addition to Cs.
3. EP 0247 414 B2 discloses, in the first claim, the prerequisite of having a carrier containing at least 0.08% and up to 2% sodium. In addition, it is taught that the silver impregnating solution should also contain Na, along with K or Cs.
By contrast, the claims of the following patents have disclosed the importance of lowering the amount of surface sodium:
1. U.S. Pat. No. 4,368,144 states that better performance is obtained with carriers that contain no more than 0.07% Na.
2. WO 00/15333, 15334, 15335 disclose improvement of the properties of the carrier by lowering the concentration of ionizable species, especially Na and silica, using boiling de-ionized water. The patents disclose that it is preferred to lower the concentration of Na and silica by at least 5%.
In the preferred Na removal method, the carrier is repeatedly immersed in boiling water.
3. U.S. Pat. No. 6,103,916, EP 0937 498A1 claim that catalyst performance is improved when the carrier is washed by boiling in pure water until the water resistivity is more than 10,000 xcexa9.cm.
Lithium has repeatedly been mentioned as an example of the alkali metals that can be added to improve the selectivity of the catalyst. It has been mentioned along with Na, K, Rb and Cs, with Cs as the preferred promoter alkali metal. In few cases, however, Li was added to Cs as a co-promoter, e.g. U.S. Pat. Nos. 4,272,443, 4,278,562, 4,212,772 and EP 0384 312 B1. Also, EP 0624 398 B1 discloses the addition of Li to the silver impregnating solution, along with the other promoters: Cs, W and Na (example 2).
Several patents have indicated that Li and Na are similar in their effect on the catalyst""s performance:
1. U.S. Pat. No. 4,916,243 discloses using combinations of a Cs salt and a salt of any of the other alkali metals.
2. U.S. Pat. No. 4,820,675 discloses using combinations of a Cs salt and a salt of any of the other alkali metals. Addition of Li to Cs was augmented with addition of Na, column 7, and experiment 7-28, column 25.
3. U.S. Pat. No. 4,212,772 indicates that Na and Li are equivalent with respect to their influence on the catalyst""s life, and selectivity, and that their mixtures, in all proportions gave favorable influence, column 2 line 49.
4. WO 00/15333, 15334, 15335 disclose improving the carrier via removing xe2x80x9cionizable speciesxe2x80x9d from its surface. These ionizable species include sodium, cesium and lithium.
Treating the carrier with Li before use is known and has been disclosed in the following cases:
1. U.S. Pat. No. 5,705,661 discloses that the carrier was pretreated by impregnation with Li and Cs provided that at least 100 ppm Li will be present in the finished catalyst. The pretreatment was based on soaking the carrier in a water solution containing both Li and Cs carbonates, followed by drying.
2. EP 0716 884 B1 discloses the preference for pre-depositing a pre-dopant of at least one alkali metal, Li, K, or Cs. The pre-doping procedure involves vacuum impregnating the carrier for three minutes and then drying the carrier at a temperature of up to 1000xc2x0 C. The amount of predopant is in the range of 10 to 5000 ppm.
3. U.S. Pat. Nos. 3,563,913 and 3,563,914 describe preimpregnation of alpha alumina with a lithium compound such as lithium hydroxide followed by drying before silver impregnation.
4. WO 00/15333 suggests washing and ion exchange, among several other methods, to lower the concentration of ionizable species, particularly silicates. Tetraethyl ammonium hydroxide, ammonium acetate, lithium carbonate and barium acetate are mentioned as examples of wash and ion exchange solutions. There are no examples that showing treatment with Li and the disclosure does not mention, or suggest the possibility of utilizing alternative lithium salts.
WO 00/15333, 15334, 15335 disclose that ionizable species to be removed from the carrier""s surface, especially the silicates are soluble in the same solutions that Na is soluble in. Therefore, measurement of the solubilization rate of Na is a direct measurement of the solubilization of the other ions (WO 00/15335, P 4, line 1)
In spite of the myriad of sometimes inconsistent teachings in the prior art, it has been discovered that carriers which have the surface sodium removed and partially replaced with lithium, in a pre-treatment process, give catalysts with improved performance, especially higher stability. This is quite distinctive from the prior art which addressed the effect of Na and Li as two separate issues.
The concentration of Na on the surface of the carrier may be higher or lower than the underlying layers. The amount of this surface Na may be influenced by the bulk""s composition and may also be a function of the composition of the binding material and the firing parameters of the carrier. Since the active silver particles are deposited only on the carrier""s surface, the chemistry of that surface influences the function of the silver and has a profound effect on the catalyst""s performance.
We have discovered that the ultimate catalyst performance for ethylene oxide production is greatly enhanced when the Na ions on the surface of the carrier are partially replaced with Li ions, in a pre-treatment step.
Replacing sodium with lithium yields a surface that has Li ions and is totally, or partially, depleted of its sodium. The general target of the treatment is to remove at least 25% of the surface sodium and at least partially replace it with lithium. It is preferred to remove at least 50% of the surface sodium and most preferred to remove at least 90% the sodium on the surface, and partially replace it with up to 10 ppm, suitably 1-10 ppm Li.
An additional feature of this invention is to control the amount of silicon compounds removed from the surface, i.e. contrary to the prior art it is essential that the sodium removal is not accompanied by a similar silicon solubilization and removal. Silicon is added as a bonding material and its removal weakens the carrier. We have discovered that removal of the silicates does not contribute to improvement in the catalytic performance.
Silica and silicates are important components of the carrier""s composition. The carrier is made essentially of alumina, silica, and/or aluminum silicate particles that are shaped into pellets and fired at high temperature. Silica, or a silicate, is added to the binding material which holds these particles in the final pelletized shape. Therefore it is expected that the surface of the carrier will contain silicon compounds and it is a feature of the instant invention, that the carrier pretreatment should not substantially remove this binding material, i.e. removal and replacement of sodium should not be associated with comparable removal of silicon compounds.
The amount of silicon compounds in the carrier can vary broadly depending upon the manufacture.
We have discovered that there is no correlation between the rate of removal of surface sodium and that of the silicates, contrary to the teaching of the prior art. Actually, we have discovered that with prolonged washing at high temperature the ratio of Si/Na removed increases and does not have a fixed value, as was claimed in the prior art. We have discovered that this ratio can be reduced by lowering the temperature of the Li treatment, i.e. at a lower temperature of the Li treatment only a minimum amount of silicates is removed. This results in a lower ratio of the removed Si/Na in solution. Therefore, according to the present invention, it is essential to conduct the support pretreatment at a temperature lower than 100xc2x0 C., preferably lower than 80xc2x0 C. and it is most preferred that the support pretreatment temperature is lower than 70xc2x0 C. In accordance with the present invention, it is advantageous that the carrier treatment is carried out such that the Si/Na weight ratio of removed material is less than about 5.0, and preferably is less than about 2.0. This is in sharp disagreement with procedures of the prior art where Si/Na ratios of removed material frequently are in excess of 10.
Utilizing pure water to remove surface sodium results in the removal of a considerable portion of the surface silicon. With pure water, the targeted Na removal value is achieved only when the water is at, or close to, its boiling point. At this temperature a large amount of silicates are also removed. On the other hand when pure water is used at a temperature considerably lower than 100xc2x0 C., the sodium removal is rather limited and does not reach the assigned target, even after repeated washing for several hours. Accordingly carrier treatment with water alone is not effective in producing the improved carrier.
The concentration of the surface Na ions of the untreated carrier is the essential target for the treatment of the invention. This concentration is determined by the carrier manufacturer using the xe2x80x9cAcid-Leachable testxe2x80x9d. In the standardized acid leachable test, the carrier sample is digested for a short period of time in 30% nitric acid solution. The sodium, potassium, calcium and silicon concentrations in the resulting solution are determined by atomic absorption spectro-photometry, Varian AA-110, in an air/acetylene flame using the wavelengths of 589.0 nm and 766.5 nm respectively. Alternatively, quantification is performed by aspirating the solutions into an inductively coupled plasma spectrophotometer, Spectro-analytical EOP ICP. The wavelengths used to simultaneously determine Al, Si, Na and K are 394.40 nm, 212.41 nm, 589.59 nm, and 766.46 nm respectively. Based on the surface Na concentration, the goal of the present invention is to remove at least 25%, preferably at least 50% and most preferably at least 90% of the sodium and replace the removed Na with up to 10 ppm Li, preferably 5 to 10 ppm Li.
The amount of sodium removed from the surface is measured through analysis of the solution used in the pretreatment.
The carrier pretreatment may be accomplished by any means which are effective. For purpose of illustration the following methods are viable means in achieving the pretreatment goal:
1. Heating the carrier in a solution which contains a lithium salt. The heating treatment continues until the targeted sodium concentration is detected in the treatment solution.
2. Stirring the carrier in a solution that contains a lithium salt, at room temperature or at an elevated temperature. The mixing continues until the targeted sodium concentration is detected in the treatment solution.
3. Pumping lithium solution over a bed that contains the carrier to be treated, at room temperature or at an elevated temperature.
4. Vacuum impregnate the carrier with the lithium solution and then wash the carrier with water.
In general, it is preferred to combine two or more of the abovementioned methods in one treatment.
The solvent suitable for the pre-treatment is a function of its ability to dissolve the lithium salts used and also to dissolve the removed sodium ions, without the concurrent excessive removal of the silicate anions. Common solvents as water, alcohol, or their mixtures are suitable for the pretreatment.
It is essential that the lithium""s counter ion, the anion of the salt, not leave a residue on the carrier surface which would interfere with the catalytic function. Examples of the suitable lithium salts are lithium chloride, carbonate, nitrate, formate, and hydroxide. Solutions suitable for the pretreatment are 0.001N to 1.0N aqueous lithium salt solution. It is preferred to use 0.005 N to 0.5 N lithium solution and most preferred to use 0.01 N to 0.1 N lithium salt solution.
We have also discovered that for optimum catalytic performance Li should not replace more than a fraction of the removed Na. Therefore, it is preferred that Li should replace not more than 50% of the removed Na (on a molar bases). It is most preferred if the Li replacement is limited to not more than 25% of the removed Na. In general, the finished catalyst will contain less than 10 ppm Li.
After the pretreatment with lithium, the carrier is dried in order to remove the solvent from the carrier""s pores, in preparation for the impregnation with the silver solution. It is however, essential to wash the treated carrier, before or after the drying step, with pure solvent before utilizing it in the preparation of the catalyst.
At the end of the pretreatment with the Li solution, the pores of the carrier will contain a solution that contains both lithium and sodium, as well as the other species that were removed from the carrier surface. Drying the carrier will lead to deposition of these materials-and will contaminate the surface. Therefore, washing the carrier with the pure solvent after the Li treatment will result in reducing the amount of surface contamination and will lead to improved performance. In place of pure solvent, a weak Li solution may be used in the final rinse.
The sodium removal treatment of the present invention is distinct from various pre-doping or pre-impregnation treatment such as described, for example, in EP 0 716 884. In the prior procedures, the lithium dopant is added to the carrier surface and deposited thereon in addition to the surface Na. In the present invention, replacement of sodium and removal thereof is essential.
Drying the carrier may be achieved in vacuum or under atmospheric pressure. The carrier is dried at a temperature lower than 400xc2x0 C., and preferably at a temperature lower than 200xc2x0 C. The carrier is most preferably heated to a temperature 0-50xc2x0 C. higher than the boiling point of the solvent until all the solvent in the pores is evaporated.
The instant invention is preferred because it provides the following unique advantages:
1. Efficiency of sodium removal:
The rate of sodium removal in the present invention is more efficient than the water washing of the prior art.
2. Targeted level of sodium removal:
The present invention sets a target for sodium removal. This targeted level is proportional to the surface concentration, as indicated by the acid leachable test. In this regard different carriers will have different targets for sodium removal. Water, as used in the prior art, has a limited capacity for sodium removal and will not be able to remove the targeted amount in a practicable procedure.
3. Controlled Lithium deposition:
The prior art discloses carrier treatment with lithium without the concurrent sodium removal. This leads to an over abundance of both alkali metals on the surface of the carrier, which will interfere with the deposition of silver and will influence the catalyst""s stability. The present invention avoids this serious disadvantage by removing sodium from the pores concurrently with the lithium deposition.
4. Removal of unbound alkali metals:
In the few cases of the prior art when the carrier was treated with Li as a pre-dopant, it was not washed after the treatment. Upon drying, the Li and sodium salts that were in solution inside the pores were deposited on the surface. This large amount of unbound salts will interfere with the catalytic function. The present invention offers a washing step after the lithium pre-treatment to remove the majority of the unbound ions. Unbound ions are those ions that are not bound to a specific site on the carrier""s surface and if left in the pores will result in salts deposition on the surface.
5. Avoids removal of silicates:
The instant invention avoids the excessive removal of the silicon compounds concurrently with Na removal. Silicates are useful for the carrier""s strength and their removal does not contributes to a better catalytic performance.
6. Lower drying temperature:
The prior art disclosed that the drying temperature of the carrier may be up to 1000xc2x0 C., preferably up to 600xc2x0 C. This high drying temperature would lead to the migration of sodium ions from the subsurface to the surface, leading to poor catalytic performance. The current invention discloses-the preference of a much lower drying temperature to avoid the migration of sodium.
7. Proof of catalyst stability
The instant invention presents comparative examples that substantiate the claimed improved performance, especially the stability of the pretreated carrier.
The catalysts of the instant invention are characterized by having higher performance stability and higher selectivity to produce ethylene oxide. As will be illustrated in the examples, the stability of these catalysts is superior to catalysts that have the lithium components added in the silver impregnation step, catalysts made with untreated carriers, carriers that were treated only with water, or carriers that were pretreated with lithium without the concurrent removal of sodium.
Preferred supports are those containing principally alpha-alumina, particularly those containing up to about 15 wt % silica. Especially preferred supports have a porosity of about 0.1-1.0 cc/g and preferably about 0.2-0.7 cc/g. Preferred supports also have a relatively low surface area, i.e. about 0.2-2.0 m2/g, preferably 0.4-1.6 m2/g and most preferably 0.5-1.3 m2/g as determined by the BET method. See J. Am. Chem. Soc. 60, 3098-16 (1938). Porosities are determined by the mercury porosimeter method; see Drake and Ritter, Ind. Eng. Chem. anal. Ed., 17, 787 (1945). Pore and pore diameter distributions are determined from the surface area and apparent porosity measurements.
For use in commercial ethylene oxide production applications, the supports are desirably formed into regularly shaped pellets, spheres, rings, etc. Desirably, the support particles may have an equivalent diameters in the range from 3-12 mm and preferably in the range of 4-10 mm, which are usually compatible with the internal diameter of the tubes in which the catalyst is placed. An Equivalent diameter is the diameter of a sphere having the same external surface (i.e. neglecting surface within the pores of the particle) to volume ratio as the support particles being employed.
Preferred catalysts are prepared in accordance with this invention contain up to about 30% by weight of silver, expressed as metal, deposited upon the surface and throughout the pores of a porous refractory support. Silver contents higher than 20% by weight of total catalyst are effective, but result in catalysts which are unnecessarily expensive. Silver contents, expressed as metal, of about 5-20% based on weight of total catalyst are preferred, while silver contents of 8-15% are especially preferred.
In addition to silver, the catalyst of the invention also contains promoters, especially a critical amount of alkali metal promoter component. The amount of the alkali metal promoter is not more than 3000 ppm expressed as alkali metal based on the catalyst weight; preferably the catalyst contains 400-1500 ppm, more preferably 500-1200 ppm alkali metal based on the catalyst weight. Preferably the alkali metal is cesium although lithium, potassium, rubidium and mixtures thereof can also be used.
Optionally also of practice of the invention is the provision of sulfur as a promoting catalyst component. The sulfur component can be added to the catalyst support impregnating solution as sulfate, eg. cesium sulfate, ammonium sulfate, and the like. U.S. Pat. No. 4,766,105 describes the use of sulfur promoting agents, for example at column 10, lines 53-60, and this disclosure is incorporated herein by reference. When used, the amount of sulfur (expressed as the element) in the amount of 5-300 ppm by weight, based on the weight of catalyst, is preferred.
The catalyst may also contain a fluorine promoter in the amount expressed as the element of 10-300 ppm by weight based on the weight of the catalyst. Ammonium fluoride, alkali metal fluoride, and the like can be used.
Preferably, the silver is added to the support by immersion of the support into a silver/amine impregnating solution or by the incipient wetness technique. The silver containing liquid penetrates by absorption, capillary action and/or vacuum into the pores of the support. A single impregnation or a series of impregnations, with or without intermediate drying, may be used, depending in part upon the concentration of the silver salt in the solution. To obtain catalyst having silver contents within the preferred range, suitable impregnating solutions will generally contain from 5-50 wt % silver, expressed as metal. The exact concentration employed will depend upon, among other factors, the desired silver content, the nature of the support, the viscosity of the liquid, and the solubility of the silver compound.
Impregnation of the pretreated carrier is achieved in a conventional manner. The carrier is placed in the silver solution until all of the solution is absorbed by the support. Most preferably, the dry pretreated carrier is placed under vacuum and then the silver solution is introduced. The vacuum is removed only when all the carrier""s pellets are coated with the solution, or when the liquid level is sufficient to cover the amount of carrier used. This ensures that all the pores of the carrier have been filled with the impregnating solution.
The impregnating solution, as already indicated, is characterized as a silver/amine solution, preferably such as is fully described in U.S. Pat. No. 3,702,259 the disclosure of which is incorporated herein by reference.
After impregnation, any excess impregnating solution is separated and the support, impregnated with silver and promoters, is calcined or activated. In the most preferred practice of the invention, calcination is carried out as described in commonly assigned U.S. Pat. No. 5,504,052 granted Apr. 2, 1996 and co-pending application Ser. No. 08/587,281 filed Jan. 16, 1996, the disclosures of which are incorporated herein by reference. The calcination is accomplished by heating the impregnated support, preferably at a gradual rate, to a temperature in the range of 200-500xc2x0 C. for a time sufficient to convert the contained silver salt to silver metal and to decompose the organic materials and remove the same as volatiles.
The impregnated support is optionally maintained under an inert atmosphere while it is above 300xc2x0 C. during the entire procedure. While not wishing to be bound by theory, it is believed that at temperatures of 300xc2x0 C. and higher, oxygen is absorbed in substantial quantities into the bulk of the silver where it has an adverse effect on the catalyst characteristics. Inert atmospheres which are optionally employed in the invention are those which are essentially free of oxygen.
An alternative method of calcination is to heat the catalyst in a stream of air at a temperature not exceeding 300xc2x0 C., preferably not exceeding 270xc2x0 C.
Catalysts prepared in accordance with the invention have improved performance, especially stability, for the production of ethylene oxide by the vapor phase oxidation of ethylene with molecular oxygen. These usually involve reaction temperatures of about 150xc2x0 C. to 400xc2x0 C., usually about 200xc2x0 C. to 300xc2x0 C., and reaction pressures in the range from 0.5 to 35 bar. Reactant feed mixtures contain 0.5 to 20% ethylene and 3 to 15% oxygen, with the balance comprising comparatively inert materials including such substances as nitrogen, carbon dioxide, methane, ethane, argon and the like. Only a portion of the ethylene usually is reacted per pass over the catalyst and after separation of the desired ethylene oxide product and the removal of appropriate purge streams and carbon dioxide to prevent the uncontrolled build up of inerts and/or by-products, unreacted materials are returned to the oxidation reactor.
The following examples illustrate the invention.